This set of experiments has revealed a remarkably rich set of flow regime transitions induced by the injection of drag reducers into a horizontal multiphase flow. The experiments were performed in a high pressure gas-condensate flow over a range of gas and liquid flow rates such that all of the two-phase flow regimes normally observed in horizontal multiphase flowlines - stratified flows, annular flows, and slug flows - were realized experimentally. Flow visualization captured the transition of slug flows to stratified flows; annular-entrained flows to annular flows (with no entrainment) or to stratified flows; stratified-wavy flows to stratified-smooth flows. These observations were used to develop a flow-pattern map for horizontal two-phase flow with drag reducers, which contrasts dramatically with the conventional flow-pattern map. The liquid volume fraction plays an important role in determining the magnitude of drag reduction in multiphase flows. The present experiments indicate that for a fixed superficial gas velocity, the higher the liquid volume fraction (i.e., the "wetter" the flow) the higher the drag reduction. Beyond a liquid volume fraction of approximately 5%, the drag reduction effectiveness no longer increases - it reaches a plateau level that can be as high as 65%. The plateau level of drag reduction decreases with increasing superficial gas velocity.
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